1. Field of the Invention
This invention relates to a rear projection display device which enables an observer to observe a picture on a front surface of a screen by slanting projecting image light onto a back surface of a screen.
2. Description of the Prior Art
FIGS. 9, 10 illustrate one example of a conventional rear projection display device. Specifically, FIG. 9 illustrates a cross sectional view of a conventional rear projection display device, and FIG. 10 illustrates a top plan view of the rear projection display device shown in FIG. 9. In the following description, a coordinate system is used where a horizontal direction of a rectangle screen 170 is taken along an x-axis, a vertical direction of the screen 170 is taken along a y-axis, and a perpendicular direction to the screen 170 is taken along a z-axis.
The rear projection display device of FIG. 9 includes: a projection unit 120 arranged in a body 110; a projection lens 130 arranged on a light emitting opening of the projection unit 120; a reflecting mirror 160 arranged on an inner back surface of the body 110; and a transmission type diffusing screen 170 arranged on the front of the body 110. Image light, which is magnified and projected from the projection unit 120 through the projection lens 130, is reflected on the reflecting mirror 160 and is irradiated onto a back surface of the diffusing screen 170. A picture is observed on the front surface of the diffusing screen 170.
As shown in FIG. 10, the projection unit 120 includes a white light source 121 comprising a lamp 121a and a reflector 121b. Dichroic mirrors 122, 123 split the white light emitted from the white light source 121 into three colors of light. A first dichroic mirror 122 selectively reflects light of a red component (referred as “red light” hereinafter) out of the white light emitted from the lamp 121a and transmits the light of the other color components. A second dichroic mirror 123 selectively reflects light of a green component (referred as “green light” hereinafter). The green light from the light transmitted through the first dichroic mirror 122 is selectively reflected on the second dichroic mirror 123 and is introduced to a liquid crystal panel 127g. Light of a blue component (referred as “blue light” hereinafter), from the light transmitted through the second dichroic mirror 123, is introduced to a liquid crystal panel 127b for the blue light by reflecting mirrors 125, 126. The red light reflected on the first dichroic mirror 122 is introduced to a liquid crystal panel 127r by the first reflecting mirror 124.
The color lights are modulated at the liquid crystal panels 127r, 127g, and 127b, respectively, and are synthesized at a dichroic prism 128 and subsequently emitted to the projection lens 130.
Incident directions of the color lights modulated at the liquid crystal panels 127r, 127g, and 127b to the dichroic prism 128 are set with the consideration of color reproducibility at the dichroic prism 128. Light reflected on the dichroic prism 128 is S-polarized light, and light transmitted through the dichroic prism 128 is P-polarized light.
The S-polarized light is a linearly polarized light wherein the oscillation direction of the electric vector of the light incident to a sample surface, is vertical to a surface including a normal of the sample surface and a normal of a wave surface which is a light traveling direction. The P-polarized light is a linearly polarized light wherein the oscillation direction of the electric vector of the light incident to a sample surface, is included in an incident surface (a surface including a normal of the sample surface and a light traveling direction).
Specifically, the red light from the light incident to the dichroic prism 128 is set to be S-polarized to a bonded surface 128x. A polarized light component, which is perpendicular to an x-z plane, is reflected on the bonded surface 128x. The green light is set to be P-polarized light to the bonded surfaces 128x, 128y. A polarized light component, which is parallel to the x-z plane, is transmitted through the bonded surface 128x, 128y. The blue light is set to be S-polarized light to the bonded surface 128y. A polarized light component, which is perpendicular to the x-z plane, is reflected on the bonded surface 128y. And then the red, green, and blue light is color-synthesized.
The color-synthesized image light is irradiated from the projection lens 130 to the back surface of the screen 170 through the reflecting mirror 160.
Recently, a rear projection display device capable of slantly irradiating image light to the screen 170 for reducing the depth of the device was proposed. When the above mentioned projection unit 120 was used for projecting an image from a slanted angle, a polarization direction of the projected image light to the screen 170 was set in the direction orthogonal with the polarization direction of the image light to the dichroic prism 128. The red light was P-polarized, the green light was S-polarized, and the blue light was P-polarized.
When the image light was projected onto the screen 170 on a slant, the light was incident to the acrylic resin from an air with a certain angle of incidence out of a vertical incidence. FIG. 6 is a table showing the reflectivity characteristics of light incident to the acrylic resin from the air. As shown in FIG. 6, when the image light was projected onto the screen 170 on a slant, the reflectivity of P-polarized light to the screen 170 was lowered while the reflectivity of S-polarized light to the screen 170 increased.
FIG. 8 presents the spectral luminous efficiency characteristics of a man. As shown in FIG. 8, the spectral luminous efficiency of a man's eyes is the highest at around a wavelength of 555 nm which corresponds to the green color, and therefore, a man is more likely to recognize green light being brighter in comparison with red and blue light.
As a result, when the image light was projected by using the conventional projection unit 120, the reflectivity of the brightest green light at the screen 120 increases, and the brightness as a whole is lowered. Furthermore, the image quality is degraded because of the reflected light.